The method of thermal dye transfer (TT) is used to reproduce a digitally produced image in the form of a printed image, of which the image quality is matched to the level of silver salt photography. The digital image is processed point by point in terms of the basic colours cyan, magenta, yellow and black and is converted into corresponding electric signals, which are then converted into heat by means of the thermal head of a printer. Due to the influence of heat, the dye sublimates from the donor layer of a colour ribbon (colour sheet) in contact with the recording material to be printed and diffuses into the receiving layer of the recording material.
In order to achieve images of photo quality, a recording material requires good surface properties, low thermal conductivity, good heat resistance, high compressibility (softness), which is important in order to ensure good contact between the thermal head of the printer and the recording material, and good dimensional stability. In addition, the recording material must have good storage life after printing in order to prevent the migration of the dyes over time into and through the carrier and therefore in order to prevent the deterioration of the image quality.
Recording materials for thermal transfer printing have been described many times. They basically comprise a carrier, a dye-receiving layer, and optionally further functional layers. The requirements placed on the material can be optimised by suitable selection of the components of the recording material.
Uncoated or coated papers can be used as carrier, wherein synthetic-resin-coated papers, coated in particular with polyolefin, or papers provided with a multi-layered plastics film are particularly suitable. These carriers are described for example in EP 0 671 281 A1, EP 0 681 922 Al or EP 0 812 699 A1.
The dye-receiving layer contains a resin that has an affinity for the dye from the donor material. By way of example, plastics comprising ester compounds (such as polyester resins, polyacrylic acid ester resins, polycarbonate resins, polyvinyl acetate resins, styrene acrylate resins), plastics comprising amide compounds (such as polyamide resins), polyvinyl chloride and mixtures of the aforementioned resins can be used for this purpose. However, copolymers that as primary component contain at least one representative of the above-mentioned polymers, for example vinyl chloride/vinyl acetate copolymer, can also be used.
Further functional layer may comprise, by way of example, anti-curl layers in order to counteract the curvature of the recording material once passed through the thermal printer. By way of example, plastics films that are laminated onto the rear side of the recording material are well suited for this purpose.
The problem of compressibility can be solved by the application of an intermediate layer performing the function of a cushioning layer, as described in JP 02-274592 or JP 03-268998. The intermediate layer described in JP 04-21488 containing hollow microbeads may constitute an alternative approach. This intermediate layer additionally has an insulating effect and contributes to a reduction of the thermal conductivity.
WO 98/10939 A1 describes a recording material for thermal image recording having good temperature stability and low thermal conductivity as well as good compressibility. This object is achieved with the aid of a layer that consists of a gelatine derivatised with ethylenically unsaturated monomers, in particular a (meth)acrylated gelatine. A disadvantage of this recording material is the high application weight of this layer. This is because layer thicknesses from 20 to 100 μm are necessary in order to achieve the required low thermal conductivity. The problem of dye migration into the depth of the recording material, however, therefore still remains unsolved. A reduction of the dye migration may therefore be attained by curing the layer in an additional process step with the aid of a smooth cylinder.